The use of tetraethylorthosilicate (TEOS) as the source of a deposited oxide is well known. Such material can be used to form a deposited oxide layer at a relatively low temperature so as not to disturb the diffusions previously made into the wafer. It is well known that a thermal oxide is much denser than a deposited oxide. However, a thermal oxide is formed at a higher temperature at which the previously made diffusions will move deeper into the semiconductor wafer. Accordingly, there is a need for forming a deposited oxide layer at a lower temperature on a semiconductor wafer itself or on such a wafer completely or partially covered with thermal oxide. It is also known that a subsequent annealing step will densify the deposited oxide layer.
The prior art process for forming deposited oxides using TEOS used nitrogen as the carrier gas. This nitrogen gas is passed over the TEOS as it resets in a source container, or the carrier gas is bubbled through the TEOS held in the source container.
The decomposition temperature of TEOS is well known and lies within a range having as its upper usable temperature, a temperature of 960.degree.C. As the temperature is reduced, the decomposition rate, as well as the vaporization rate, is reduced until a point at which the chemical reaction stops or the chemical reaction is carried on so slowly that it appears to stop. The use of a particular temperature can be selected by a man skilled in the art for both the prior art process as well as the process of the invention.
In the use of the prior art process, the wafers to be covered with a deposited oxide film are laid broadside down in a furnace boat. The difficulties of obtaining acceptable results require that the wafers be placed in a line extending from the source input end of the furnace tube to the source output end of the furnace tube and lying directly in the path of the movement of the source gas. These constraints are required to achieve something of a uniform thickness of deposited oxide over a significant portion of the wafer surface. In practice, it has been found that seven to ten wafers can be placed in such a line and that the deposited oxide will be formed on such wafers in a stripe covering less than the entire surface of the wafers.
Wafers processed according to the above-described steps do not always result in layers having acceptable characteristics. One such characteristic which has been a source of defects in semiconductor devices subsequently made in such wafers is the presence of impurities in the film. Such impurities result in a film which is insufficiently dense or which cause differences in the electrical characteristics between devices fabricated on such a wafer.
In the design and fabrication of semiconductor devices, a predetermined thickness of a deposited oxide layer is required. This required thickness has established upper and lower limits for producing a minimum number of workable devices. In the prior art, the target thickness is identified at 1500 Angstroms and the variation can be .+-.500 Angstroms. With this as a goal to work towards, it is important to note that in the use of the prior art process, not only was the deposited oxide formed on the wafers over a stripe which is less than the total surface area of the wafer, but the variation in thickness from side to side perpendicular to the gas flow can be as high as 500-2000 Angstroms. The variation from one edge to the opposite edge parallel to the gas flow is less than that achieved side to side and usually the difference in this direction was not such a variation as to render the active devices formed in this area unacceptable due to thickness of the deposited oxide layer.
While the variation of deposited oxide, from edge to edge in the direction parallel to the flow of the source gas, and more particularly, in the center of the wafer, does not experience as great a variation as is the variation perpendicular to the gas flow, there is a problem in achieving acceptable thicknesses in this direction. When the decomposed TEOS contacts the first wafer in its path, it tends to form a silicon dioxide layer at the point of first contact. Hence, with a minimum TEOS flow all the material could deposit out of a gas flow before reaching the last wafer in the line. In this manner with an insufficient gas flow, the first wafer would have a greater thickness on its surface, while the last wafer could have no coverage at all. In order to prevent this from happening, a greater gas flow is achieved which pushes the source material to the end of the tube so that the wafer at the exhaust end of the tube is also covered with a thickness comparable with the thickness on the first wafer in the tube. Many factors enter the achievement of this result; namely: the temperature of the source material, the temperature of the furnace, the geometric design of the source bottle, the gas flow through the source material as well as all other geometric dimensions as to pipes, openings, etc.
The prior art teaches increasing of flow of source material to that flow at which the flow down the tube is high enough that the deposited silicon dioxide material forms in the stripe down the central portion of the wafers, as previously discussed.